Low corrosive integrated process for preparing dialkyl carbonates

ABSTRACT

An integrated process for the production of a dialkyl carbonate and a diol from an alkylene oxide, carbon dioxide and an aliphatic monohydric alcohol is described in which an alkylene oxide is first reacted with carbon dioxide in the presence of a halogen-free carbonation catalyst to provide a corresponding cyclic carbonate and the cyclic carbonate is then reacted with an aliphatic monohydric alcohol in the presence of the carbonation catalyst and/or a transesterification catalyst and recycling the carbonation catalyst to provide a corresponding dialkyl carbonate and diol, wherein the dialkyl carbonate product exhibits a halogen concentration of about 5 ppm or less.

[0001] This is a continuation-in-part of U.S. patent application Ser.No. 09/887,642, filed on Jun. 22, 2001.

[0002] This invention relates to a low corrosive process for preparingdialkyl carbonates and diols. More specifically, the present inventionrelates to an integrated process for preparing dialkyl carbonates anddiols from alkylene oxides, carbon dioxide and alcohols having achlorine concentration of 5 ppm or less, preferably 2 ppm or less.

BACKGROUND OF THE INVENTION

[0003] Dialkyl carbonates are important intermediates for the synthesisof fine chemicals, pharmaceuticals and plastics and are useful assynthetic lubricants, solvents, plasticizers and monomers for organicglass and various polymers, including polycarbonate, a polymer known forits wide range of uses based upon its characteristics of transparency,shock resistance and processability.

[0004] Industrially, dimethyl carbonate (DMC) is used in the productionof polycarbonates and has the potential to be used as an environmentallyfriendly fluid for numerous solvent-related applications and conceivablyeven as a fuel oxygenate (e.g., methyl tertiary butyl etherreplacement).

[0005] Historically, DMC was prepared from the highly toxic intermediatephosgene, COCl2. Currently, it is prepared via oxidative carbonylationof methanol using a copper(I) chloride catalyst together with a halogenmitigation step using HCl. This method is based on copper(I) chloride asthe catalyst and demonstrated in EP 534,545 B1 and EP 460,732 A1. Theoverall copper catalyzed reaction is shown in equation (1) below:

CO+2CH₃OH+½O₂→(CH₃O)₂CO+H₂O  (1)

[0006] The copper(I) chloride catalyst is very insoluble in this systemand, thus, is a limiting component in the catalytic cycle. Hydrochloricacid is also added as a component in this oxidative carbonylation systemduring a mitigation step. This was done to prevent the oxidation ofCu(I) to Cu(II) in the presence of oxygen and water since Cu(I) isbelieved to be the active species in this system. This copper chloridecatalyst-based oxidative carbonylation system, which is run between 120°C. and 160° C., is extremely corrosive and requires costly components(e.g., glass lined reactors). Failure in the glass lining could lead torupture or explosion. Two other notable processes for the production ofDMC are disclosed in U.S. Pat. Nos. 6,010,976 and 5,498,744. U.S. Pat.No. 6,010,976 discloses a catalytic reaction of urea with methanol tofirst form the carbamate, which is further reacted to form DMC, ammoniaand carbon dioxide. U.S. Pat. No. 5,498,744 discloses a process thatreacts methylnitrite with carbon monoxide over a catalyst to form DMCand (NO)x which is toxic.

[0007] DMC, due to its low toxicity and low atmospheric reactivity, hastremendous growth potential as a possible replacement for methyltertiary butyl ether (MTBE), as a fluorocarbon solvent replacement inthe electronics industry and as an environmentally friendly solvent foruse in the production of polycarbonates. The problems with MTBE andfluorocarbons, and phosgene are widely publicized. The growth of DMC usehas been, in part, limited by the difficulties in commercial production.An efficient and environmentally friendly method for the large-scaleproduction of DMC would be highly desirable, especially a process thateliminates the need for a chloride-based catalyst and hydrochloric acidmitigation, which causes corrosion of the reaction vessel and impuritiesin the resultant product.

[0008] Accordingly, Applicants have developed an improved low corrosiveprocess for the production of alkyl carbonates, and, in particular, DMC,starting from carbon monoxide, oxygen and alcohol in the presence of atriesterification catalyst, wherein the halogen (e.g., chlorine)concentration of the alkyl carbonate product is 5 ppm or less.

SUMMARY OF THE INVENTION

[0009] According to the present invention, it has now been found that adialkyl carbonate and a diol, and more specifically dimethyl carbonateand ethylene glycol, can be prepared according to an integrated processhaving high productivity which uses a halogen-free carbonation catalyst,by:

[0010] reacting an alkylene oxide (ethylene oxide in the case ofdimethyl carbonate and ethylene glycol) with carbon dioxide in thepresence of a halogen-free carbonation catalyst in a first reaction zoneat a temperature in the range of about 50° C. to 250° C. and at apressure of at least about 200 psi to provide a crude cyclic carbonatestream containing a cyclic carbonate (e.g., ethylene carbonate in thecase of dimethyl carbonate and ethylene glycol) and the carbonationcatalyst; and

[0011] reacting the cyclic carbonate (e.g., ethylene carbonate) from thecrude cyclic carbonate stream with an aliphatic monohydric alcohol(e.g., methanol in the case of dimethyl carbonate and ethylene glycol),in the second reaction zone in the presence of the carbonation catalystin the crude cyclic carbonate stream to provide a crude product streamcontaining a dialkyl carbonate (e.g., dimethyl carbonate) and diol(e.g., ethylene glycol). The crude product stream preferably having ahalogen concentration of about 5 ppm or less, more preferably about 2ppm or less.

[0012] In another aspect of the present invention, it has now been foundthat a dialkyl carbonate and a diol, and more specifically dimethylcarbonate and ethylene glycol, can be prepared according to anintegrated process having high productivity by using both a halogen-freecarbonation catalyst and a transesterification catalyst, by:

[0013] reacting an alkylene oxide (ethylene oxide in the case ofdimethyl carbonate and ethylene glycol) with carbon dioxide in thepresence of a halogen-free carbonation catalyst in a first reaction zoneto provide a crude cyclic carbonate stream containing cyclic carbonate(ethylene carbonate in the case of dimethyl carbonate and ethyleneglycol) and the carbonation catalyst; and

[0014] reacting at least a portion of the cyclic carbonate (e.g.,ethylene carbonate) from the crude cyclic carbonate stream with analiphatic monohydric alcohol (methanol in the case of dimethyl carbonateand ethylene glycol), in a second reaction zone in the presence of atransesterification catalyst to provide a crude product streamcontaining a dialkyl carbonate (e.g., dimethyl carbonate) and diol(e.g., ethylene glycol). The crude product stream preferably having ahalogen concentration of about 5 ppm or less, more preferably about 2ppm or less.

[0015] Additional objects, advantages and novel features of theinvention will be set forth in part in the description and exampleswhich follow, and in part will become apparent to those skilled in theart upon examination of the following, or may be learned by practice ofthe invention. The objects and advantages of the invention may berealized and attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic of a preferred embodiment of the integratedprocess, which utilizes a circulating halogen-free carbonation catalyst;

[0017]FIG. 2 is a schematic of an embodiment of the integrated process,which utilizes a circulating halogen-free carbonation catalyst and aheterogeneous transesterification catalyst;

[0018]FIG. 3 is a schematic representation of a reaction process for theproduction of polycarbonate from ethylene utilizing the integratedprocess according to FIGS. 1 and 2 above;

[0019]FIG. 4 is a schematic representation of a transesterificationdistillation tower with upper and lower external reaction zones withintegrated heat exchangers at each reaction zone; and

[0020]FIG. 5 is a schematic representation of a transesterificationdistillation tower with upper and lower external reaction zones having abottoms recycle assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] The present invention is a continuous integrated process forpreparing low corrosive dialkyl carbonates and diols from alkyleneoxides, carbon dioxide and aliphatic monohydric alcohols, wherein theresultant dialkyl carbonates and diols exhibit a halogen (i.e.,chlorine) concentration of about 5 ppm or less, more preferably about 2ppm or less.

[0022] In preparing the dialkyl carbonates and diols, an alkylene oxideis first reacted with CO₂ in the presence of a halogen-free carbonationcatalyst (e.g., [1,1′(1−butylbenzimidazol-2yl)pentane]copper(II)di(trifluoromethanesulfonate), or hydroxides, carbonates or bicarbonatesof quaternary ammonium bases) to provide a corresponding cycliccarbonate. Preferably, the alkylene oxide is represented by structuralformula set forth below in equation (I). Examples of such alkyleneoxides include ethylene oxide, propylene oxide, styrene oxide,trimethylene oxide, cyclohexene oxide, and the like. Of these alkyleneoxides, ethylene oxide and propylene oxide are preferably used becauseof their good availability and high demand end products. Ethylene oxideis most preferably used. The alkylene oxide feed may contain variousimpurities, especially the impurities resulting from its manufacture.For instance, ethylene oxide which is made by selective oxidation ofethylene may contain carbon dioxide, water and aldehydes.

[0023] Preferred cataysts are the quaternary ammonium compounds havingthe following structural formula:

[0024] in which R₁, R₂, R₃ and R₄ are the same or different, are alkyl,aralkyl, alkenyl (a monovalent radical containing a double bondincluding, for example, allyl and vinyl) or aminoalkyl groups containingfrom 1 to 20 carbon atoms, the sum of the carbon atoms R₁, R₂, R₃ and R₄is not less than 4 and not more than 40, Y is a hydroxide carbonate orbicarbonate radical and the value of n is equal to the valence of Y, andquaternary ammonium compounds having the following structural formula:

[0025] in which R₁, R₂, n and Y have the meaning noted above and r is adivalent radical as follows:

[0026] Examples of quaternary ammonium compounds suitable as catalystsin accordance with this invention and having a structural formulacorresponding to the first formula above given are trimethyl benzylammonium hydroxide, tetraethyl ammonium hydroxide, trimethyl cetylammonium hydroxide, trimethyl butyl ammonium hydroxide, tetrabutylammonium hydroxide, diethyl diamyl ammonium hydroxide, other tetraalkylammonium hydroxides in which the alkyl groups are the same or differentand each alkyl group contains from 1 to 20 carbon atoms, and thecorresponding carbonates and bicarbonates of the above enumeratedcompounds.

[0027] Examples of quaternary ammonium compounds having a structuralformula corresponding to the second formula above given are methyl ethylpiperidinium hydroxide, methyl decyl piperidinium hydroxide, 4,4-benzylmethyl morpholinium hydroxide, 4,4-dially morpholinium hydroxide,4,4-methyl hexyl morpholinium hydroxide, 4,4-ethyl butyl morpholiniumhydroxide, 4,4-diethyl thiomorpholinium hydroxide, other dialkylpiperidinium, pyrrolidinium, morpholinium, and thiomorpholiniumcompounds.

[0028] N,N,N,N′N′N′hexamethyl-ethylene-bis-ammonium hydroxide, carbonateand bicarbonate are also effective catalysts. The formula for thehydroxide is:

(H₃C)₃N—CH₂—CH₂—N—(CH₃)₃(OH)₂

[0029] The quaternary ammonium compound may be obtained as such from anyavailable source or produced in any desired manner as set forth in U.S.Pat. No. 2,873,282, which is incorporated herein by reference.

[0030] The first carbonation reaction involving this preferred alkyleneoxide may be represented by the following:

[0031] wherein R₁ and R₂ independently of one another denote a divalentgroup represented by the formula —(CH₂)_(m)—, wherein m is an integerfrom 1 to 3, which is unsubstituted or substituted with at least onesubstituent selected from the group consisting of C₁-C₁₀ alkyl group anda C₆-C₁₀ aryl group, wherein R₁ and R₂ can share the same substituent.

[0032] The carbon dioxide to be employed can contain inert gases, suchas nitrogen, hydrogen, carbon monoxide and lower hydrocarbons, and canoriginate from natural sources or industrial gases or waste gases. Thewater content of the carbon dioxide is preferably below 1 mol %, and theconcentration of sulfur is preferably below 100 ppm by weight.

[0033] The content and amount of carbon dioxide will depend on thereaction rate, reactor type and specific catalyst used, and is adjustedto maximize the economics of the process. Preferably, the molar ratio ofalkylene oxide to carbon dioxide is about 1:1, but an excess of carbondioxide is also contemplated. Therefore, according to the presentinvention, the molar ratio of alkylene oxide to carbon dioxide ispreferably in the range from about 1:0.9 to 1:15 and more preferably inthe range from about 1:1 to 1:3.

[0034] In one aspect of the present invention a halogen-free carbonationcatalyst is used in both reaction zones of the process. In the firstreaction zone of the first aspect, the reactants (i.e., alkylene oxideand carbon dioxide) are contacted in the presence of the halogen-freecarbonation catalyst.

[0035] The catalyst can be introduced to the reactor as part of arecycle stream, the fresh feed, make-up or a combination of these. Theamount of catalyst measured as the concentration of catalyst in thereactor effluent is generally about 0.05 to 5% by weight, preferablyabout 0.15 to 2.0% by weight.

[0036] In the first aspect, the carbonation reaction is preferablycarried out in a continuous mode utilizing various reactionconfigurations, such as a stirred-tank, tubular, fixed or packed-bedreactor, in a single or multiple-reactor configuration, at from about50° C. up to about 250° C., preferably between about 100° C. up to about200° C. and more preferably between about 150° C. up to about 200° C.,and at pressures ranging from about at least 1379 kPa (200 psi) up toabout 13790 kPa (2000 psi), preferably from about 2069 kPa (300 psi) upto about 8964 kPa (1300 psi) and more preferably from about 3448 kPa(500 psi) up to about 6895 kPa (1000 psi). In the preferred mode ofoperation, the reactor temperature and pressure are optimized to ensurea relatively high conversion and selectivity to the desired alkylenecarbonate. A provision for heat removal from the reactor is normallyrequired, since the carbonation reaction is exothermic.

[0037] Preferably, the effluent from the carbonation reaction zone isfed into a second carbonation reaction zone that can operate underdifferent conditions or a different configuration to provide a greateroverall conversion of the alkylene oxide, preferably greater than 90%overall conversion. Preferably, the second carbonation reaction zone isa separate tubular polishing reactor which operates at from about 50° C.up to about 250° C., preferably between about 100° C. up to about 200°C. and at pressures ranging from at least about 1379 kPa (200 psi) up toabout 13790 kPa (2000 psi), preferably from about 2069 kPa (300 psi) toabout 8964 kPa (1300 psi).

[0038] Typically, impurities are formed in the carbonation reaction inthe form of by-products. Glycols may be formed along with the alkylenecarbonates, especially if there is water present in the system. Forexample, by reacting ethylene oxide with C02 to produce ethylenecarbonate, typically ethylene glycol and some higher molecular weightglycols are produced.

[0039] The carbonation reactor effluent, either from the firstcarbonation reaction zone or from the second carbonation reaction zone(if used), is preferably subjected to a flash separation to remove thevolatiles, such as unreacted CO2 and alkylene oxide.

[0040] The effluent from the carbonation reaction is preferably notsubjected to any further separation. Thus, the cyclic carbonate and theby-product impurities produced in the carbonation reaction, along withthe homogeneous catalyst, are then fed to the transesterificationreactor, where the cyclic carbonate is reacted with an aliphaticmonohydric alcohol in the presence of the homogeneous carbonationcatalyst to provide a corresponding dialkyl carbonate and diol.

[0041] Preferably, the aliphatic monohydric alcohol has a boiling pointlower than that of the produced diol. The type of an aliphaticmonohydric alcohol which can be used in the present invention variesdepending on the particular cyclic carbonate produced by the carbonationreaction. Examples of such aliphatic monohydric alcohols includemethanol, ethanol, n-propanol, isopropanol, alkyl alcohol, butanol(including isomers of butanol), 3-butene-1-ol, amyl alcohol (isomers),hexyl alcohol (isomers), heptyl alcohol (isomers), octyl alcohol(isomers), nonyl alcohol (isomers), decyl alcohol (isomers), undecylalcohol (isomers), dodecyl alcohol (isomers), cyclopentanol,cyclohexanol, cycloheptanol, cyclooctanol, methylcyclopentanol(isomers), ethylcyclopentanol (isomers), methylcyclohexanol (isomers),ethylcyclohexanol (isomers), dimethylcyclohexanol (isomers),diethylcyclohexanol (isomers), phenylcyclohexanol (isomers), benzylalcohol, phenethyl alcohol (isomers), phenylpropanol (isomers), and thelike. The above mentioned aliphatic monohydric alcohol may besubstituted with at least one substituent, such as a halogen atom, alower alkoxy group, a cyano group, an alkoxycarbonyl group, anaryloxycarbonyl group, an acyloxy group, a nitro group or the like.

[0042] Of the aliphatic monohydric alcohols, an alcohol having 1 to 6carbon atoms is preferably used. When ethylene carbonate is the cycliccarbonate, an alcohol having 1 to 4 carbon atoms, i.e., methanol,ethanol, propanol (isomers) or butanol (isomers) is preferably used. Themethod of the present invention can be employed advantageouslyespecially when methanol and ethylene carbonate are used as feedstocksfor the transesterification reaction.

[0043] According to the present invention, it has now been found that itis unnecessary to purify the cyclic carbonate or separate thecarbonation catalyst to achieve relatively high yields and selectivityto the desired dialkyl carbonate and diol, resulting in significanteconomic benefits and advantage due to the elimination of the separationand purification steps, e.g., one or more evaporators and two vacuumdistillation columns, previously thought necessary. In addition to loweroperating and capital costs associated with eliminating these steps, ayield benefit is realized by eliminating losses of cyclic carbonateattributable to the separation and purification steps.

[0044] As such, an integrated process is provided which produces both adialkyl carbonate and a diol with high productivity. In accordance withthe present invention, the term “productivity” means the yield per unitvolume of both the carbonation and transesterification zones per unittime, i.e., the space time yield for the overall integrated process.

[0045] This transesterification reaction may be represented by thefollowing:

[0046] wherein R₁ and R₂ independently of one another denote a divalentgroup represented by the formula —(CH₂)_(m)—, wherein m is an integerfrom 1 to 3, which is unsubstituted or substituted with at least onesubstituent selected from the group consisting of C₁-C₁₀ alkyl group anda C₆-C₁₀ aryl group, wherein R₁ and R₂ can share the same substituent;and R₃ is a monovalent aliphatic C₁-C₁₂ hydrocarbon group which isunsubstituted or substituted with at least one substituent selected fromthe group consisting of a C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group.Other components in the feed to the transesterification reactor mayinclude various other species, commonly hydroxyalkyl carbonates anddialkyl carbonates, as fresh feed or in one or more recycle streams.

[0047] The reactants to the transesterification reaction (i.e., thecyclic carbonate and the aliphatic monohydric alcohol) are contacted inthe presence of the carbonation catalyst from the carbonation reaction.The transesterification reaction is preferably carried out in acontinuous mode utilizing various reactor configurations, such as,stirred-tank or tubular reactors, in a single or multiple-reactorconfiguration, or a reactive distillation column, at from about 50° C.up to about 250° C., preferably between about 75° C. up to about 170°C., and at pressures ranging from about atmospheric pressure up to about13790 kPa (2000 psi), preferably from about 138 kPa (20 psi) up to about2069 kPa (300 psi). In the preferred mode of operation, the reactortemperature and pressure are optimized to ensure a relatively highconversion and selectivity to the desired dialkyl carbonate and diol andto optimize the economics of the overall integrated process. Generally,a reactive distillation column will tend to give higher conversions ofethylene carbonate and methanol.

[0048] In the second aspect of the present invention a halogen-freecarbonation catalyst is used in the first carbonation reaction zone anda heterogeneous transesterification catalyst is used in the secondtransesterification reaction zone. This aspect is similar to the firstaspect except for the following process conditions which are preferredfor the two catalyst process of the second aspect of the presentinvention.

[0049] In the second aspect, the reactants (i.e., alkylene oxide andcarbon dioxide) are contacted in the presence of a carbonation catalyst.In the second aspect of the present invention (i.e., thecarbonation-transesterification catalyst process) the carbonationcatalyst is preferably [1,1′(1−butylbenzimidazol-2yl)pentane]copper(II)di(trifluoromethanesulfonate), or hydroxides, carbonates or bicarbonatesof quaternary ammonium bases.

[0050] The halogen-free carbonation catalyst can be introduced to thereactor as part of a recycle stream, the fresh feed, make-up or acombination of these. The amount of catalyst measured as theconcentration of catalyst in the reactor effluent is generally about0.05 to 5% by weight, preferably about 0.15 to 2.0% by weight.

[0051] In the second aspect, the carbonation reaction is preferablycarried out in a continuous mode utilizing various reactionconfigurations, such as a stirred-tank, tubular, fixed or packed-bedreactor, in a single or multiple-reactor configuration, at from about50° C. up to about 250° C., preferably between about 100° C. up to about200° C., and at pressures ranging from about atmospheric pressure up toabout 13790 kPa (2000 psi), preferably from about 2069 kPa (300 psi) upto about 8964 kPa (1300 psi). In the preferred mode of operation, thereactor temperature and pressure are optimized to ensure a relativelyhigh conversion and selectivity to the desired alkylene carbonate. Aprovision for heat removal from the reactor is normally required, sincethe carbonation reaction is exothermic.

[0052] Preferably, the effluent from the carbonation reaction zone ofthe second aspect is fed into a second carbonation reaction zone thatcan operate under different conditions or a different configuration toprovide a greater overall conversion of the alkylene oxide, preferablygreater than 90% overall conversion. Preferably, the second carbonationreaction zone is a separate tubular polishing reactor which operates atfrom about 50° C. up to about 250° C., preferably between about 100° C.up to about 200° C. and at pressures ranging from about atmosphericpressure up to about 13790 kPa (2000 psi), preferably from about 2069kPa (300 psi) to about 8964 kPa (1300 psi).

[0053] According to the second aspect of the present invention, it hasnow been found that it is unnecessary to purify the cyclic carbonate orseparate the halogen-free carbonation catalyst to achieve relativelyhigh yields and selectivity to the desired dialkyl carbonate and diol,resulting in significant economic benefits and advantage due to theelimination of the purification steps, e.g., one or more evaporators andtwo vacuum distillation columns, previously thought necessary. Inaddition to lower operating and capital costs associated witheliminating the purification steps, a yield benefit is realized byeliminating losses of cyclic carbonate attributable to the purificationsteps.

[0054] In the second aspect, the reactants to the transesterificationreaction (i.e., the cyclic carbonate and the aliphatic monohydricalcohol) are contacted in the presence of a heterogeneoustransesterification catalyst. The transesterification catalyst cantypically include any heterogeneous catalyst known in the art whichprovides adequate reaction kinetics in the presence of the carbonationcatalyst and minimizes side reactions with the impurities contained inthe cyclic carbonate. Examples of such catalysts include ion-exchangers,such as, anion-exchange resins having tertiary amino groups, amidegroups, or at least one type of ion-exchange group selected from thegroup consisting of sulfonate, carboxylate and phosphate groups;strongly basic solid anion-exchangers having quaternary ammonium groupsas ion-exchange groups and the like; inorganic metal oxides; solidinorganic compounds, such as, silica, alumina, magnesia and transitionalaluminas, such as, pseudoboehmite, silica-alumina, silica-magnesia,aluminosilicate, gallium silicate, various types of zeolites, varioustypes of metal-exchanged zeolites, ammonium-exchanged zeolites,inorganic solid support catalysts containing metals, and the like. Theterm “transitional” means it is not fully calcined.

[0055] Preferred transesterification catalysts used in the second aspectinclude anion-exchange resins having tertiary amine, quaternaryammonium, sulfonic acid or carboxylic acid functional groups; solidinorganic compounds, such as, alumina or pseudoboehmite; solid supportcatalysts containing alkaline earth metal halides, such as, thosedescribed in U.S. Pat. No. 5,498,743, which is incorporated herein byreference; or inorganic solid support catalysts containing ions, metals,compounds or complexes of at least one element of Groups 1, 2, 4-10, 12and 13-17 (IUPAC classification, previously Groups 1A, 2A, 4B-8B, 2B and3A-7A) of the Periodic Table. Particularly, preferredtransesterification catalysts are or hydroxides, carbonates orbicarbonates of quaternary ammonium bases.

[0056] The transesterification reaction of the second aspect ispreferably carried out in a continuous mode utilizing various reactorconfigurations, such as, fixed or packed-bed reactors, in a single ormultiple-reactor configuration, or a reactive distillation column, atfrom about 50° C. up to about 250° C., preferably between about 75° C.up to about 140° C., and at pressures ranging from about atmosphericpressure up to about 13790 kPa (2000 psi), preferably from about 138 kPa(20 psi) up to about 2069 kPa (300 psi). In the preferred mode ofoperation, the reactor temperature and pressure are optimized to ensurea relatively high conversion and selectivity to the desired dialkylcarbonate and diol and to optimize the economics of the overallintegrated process. Generally, a reactive distillation column will tendto give higher conversions of ethylene carbonate and methanol, while apacked-bed reactor offers flexibility in handling various heterogeneouscatalysts.

[0057] The first embodiment of the integrated process, which utilizes acirculating homogeneous catalyst, is shown schematically in FIG. 1.Equipment not essential to the understanding of the invention, such as,heat exchangers, pumps, compressors and the like are not shown.

[0058] Referring now to FIG. 1, the carbonation reactor 1 is preferablya stirred tank reactor in which the alkylene oxide is reacted with CO2to form alkylene carbonate. The reactor 1 is charged with alkyleneoxide, catalyst and recycled ethylene carbonate via line 2 and with CO2via line 3. It should be noted that the catalyst is mainly dissolved inthe recycled ethylene carbonate, with a temporarily high localconcentration of ethylene oxide. In the case of ethylene carbonate, thereaction of ethylene oxide and CO2 is exothermic and the temperature ofthe reaction zone is usually maintained below about 250° C. and thepressure is maintained in the range from about 500 to about 1000 psia toenhance product quality, yield and selectivity. Preferably, the reactiontemperature is between about 150° C. and 200° C. The molar ratio of CO2to ethylene oxide is generally maintained at about 1.3:1 to 1:1,preferably 1.15:1 to 1.05:1. Preferably, the effluent from reactor 1 isfed to a tubular polishing reactor 4, to obtain greater than 90% overallconversion of the alkylene oxide.

[0059] The carbonation reactor effluent is withdrawn from reactor 4 vialine 5. The carbonation reactor effluent 5 contains cyclic carbonate,unreacted CO2, a small amount of unreacted alkylene oxide, halogen-freecarbonation catalyst, and by-product impurities, such as, mono- andpoly-glycols. Also provided on reactor 1 is vent line 6 which can beoperated continuously or intermittently to purge the reactor of volatileimpurities which could unfavorably affect product quality. For example,in the case of ethylene carbonate synthesis from ethylene oxide and CO2,acetaldehyde is formed which, if it remained in the reaction mixture,could initiate side reactions to form unwanted polymeric materials orother byproducts that could unfavorably affect product quality.

[0060] The carbonation reactor effluent is fed from line 5 intoseparator 7 from which CO2 and unreacted alkylene oxide is separated asa gaseous effluent and, optionally, returned to the carbonation reactor1 via lines 8 and 3. A purge line 9 is also provided to vent some or allof the overhead gas from separator 7. Separator 7 is preferably a simpleflash unit. In the case of ethylene carbonate the reactor effluent isflashed at pressures ranging from about 0.5 up to about 30 psia andtemperatures between about 120° C. and 200° C. The liquid effluent willtypically contain about 0.1 to 5 wt % catalyst, about 0.3 to 20 wt %polyglycols, about 0.2 to 20 wt % mono-ethylene glycol and about 90 to99 wt % ethylene carbonate.

[0061] The liquid effluent from separator 7 is passed via line 10 into atransesterification reactor 11, which is preferably a tubular reactor ora stirred tank followed by a tubular section. An aliphatic monohydricalcohol is also fed to transesterification reactor 11 via line 12. Themolar ratio of alcohol to cyclic carbonate fed to reactor 11 isgenerally from about 2:1 to about 6:1, preferably about 3:1 to about4:1. In the case of dimethyl carbonate and ethylene glycol, the reactionof ethylene carbonate and methanol will be maintained at a temperatureof about 80° C. to 200° C., preferably about 100° C. to 150° C., andpressures about 690 kPa (100 psi) to 2069 kPa (300 psi). The conversionper pass of ethylene carbonate to dimethyl carbonate is about 30 to 70%,preferably about 50 to 70%, most preferably about 50 to 65%.

[0062] The transesterification reactor effluent is withdrawn fromreactor 11 via line 13. The transesterification reactor effluent 13 willtypically contain dialkyl carbonate, a diol, unreacted cyclic carbonate,unreacted alcohol, homogeneous catalyst and by-products, such as,organic oxygenates and polyglycols. However, the composition, andby-product yields, in particular, can vary widely based upon thespecific catalyst and operating conditions employed.

[0063] The transesterification reactor effluent is fed from line 13 intoa distillation column 14, where an overhead product stream containingthe dialkyl carbonate, alcohol and organic oxygenates is removed vialine 15 and a bottoms product stream containing the diol, cycliccarbonate, halogen-free carbonation catalyst and polyglycols is removedvia line 16. In the case of dimethyl carbonate and ethylene glycol, thedistillation column is typically operated at a pressure of between about5 and 30 psia and a temperature range at the top of the column of about50° C. to 90° C. Optionally, a side-stream 17 is withdrawn from column14, which is depleted of the diol and cyclic carbonate, and recycled totransesterification reactor 11 to reduce the load on dialkyl carbonateproduct distillation column 18.

[0064] Alternatively, as shown in FIGS. 4 and 5, transesterificationreactor 11 and distillation column 14 can be replaced with a singledistillation tower 34 comprising a plurality of reaction zones 35 and 37communicating with a plurality of heat exchangers 36 and 38 via conduits39 and 40, respectively. Such a configuration allows reaction zones 35and 37 to operate at different temperatures than distillation tower 34.With the addition of a pressure-reducing device 42, reaction zone 37 canalso operate at a different pressure.

[0065]FIG. 5 depicts another embodiment using distillation tower 34wherein the bottoms are recycled via conduit 44 and reaction zone 35 toa lower portion of distillation tower 34. The upper reaction zone 37 isa once through process wherein alkylene carbonate (e.g., ethylenecarbonate) is fed via conduit 10 and an alkanol (e.g., methanol) is feedvia conduit 46. Bottoms can also be transported to knock-out drum 48 viaconduit 49, wherein the vapor from knockout drum 48 is recycled to tower34 and the bottoms of knock-out drum 48 sent via conduit 16 forsubsequent downstream treatment. The configurations shown in FIGS. 4 and5 can allow greater conversion of alkylene carbonate than would beobtained in a single reactor.

[0066] The overhead product stream is fed via line 15 to dialkylcarbonate product distillation column 18, where the alcohol is takenoverhead and recycled via lines 19 and 12 to transesterification reactor11 and dialkyl carbonate product is removed from the bottom of reactor11 via line 20 and sent to storage. A purge stream 21 is also providedto prevent the accumulation of light by-product impurities. In the caseof a dimethyl carbonate, the dialkyl carbonate product distillationcolumn 18 is typically operated at a pressure of about 828 kPa (120psia) to 1379 kPa (200 psia) and a temperature range for about 120° C.to 190° C. Dimethyl carbonate and methanol form a low-boiling azeotrope,so that the overhead stream includes up to about 15 wt %, and typicallyabout 5-15 wt %, dimethyl carbonate. This dimethyl carbonate is recycledto transesterification reactor 11 along with the methanol via conduits21, 19 and 12.

[0067] The bottoms product stream from distillation column 14 is fed viaconduit 16 to diol product distillation column 22, where the diolproduct is removed overhead via conduit 23 and sent to storage or forfurther processing, and a bottoms stream containing unreacted cycliccarbonate, halogen-free carbonation catalyst, polyglycols and otherheavies is removed via conduit 25. Optionally, the diol product which istaken overhead via conduit 23 may be withdrawn from an intermediatepoint in column 22, and the overhead product from column 22 is returnedto an intermediate feed point in column 14. A hydrolysis reactor canalso be incorporated into the integrated process to provide a highlypurified diol, e.g., ethylene glycol. The feed to this hydrolysisreactor can include the diol product stream, which may contain smallamounts of cyclic carbonate, and some or all of the unreacted cycliccarbonate containing stream, that is otherwise recycled to thetransesterification reactor. Water is also fed to the hydrolysis reactorto convert cyclic carbonate to diol with the formation of CO2by-product. In the case of ethylene glycol, distillation column 22 isoperated in a temperature range of about 100° C. to 170° C., under avacuum in the range of about 50 to 200 mm Hg. A portion of this bottomsstream is recycled to the carbonation reactor 1 via conduits 24 and 2.Purge conduit 25 is provided to prevent accumulation of polyglycols andother heavies. The heavies stream from purge conduit 25 may be subjectedto vacuum evaporation or distillation to recover valuable ethylenecarbonate. Make-up and recycled halogen-free carbonation catalyst is fedvia conduits 26 and 24, respectively, into conduit 2. A portion of thebottoms stream is also recycled to the transesterification reactor 11via lines 24, 27, and 10. The proportion of the bottoms stream which isrecycled to each reactor 1 and 11 will be chosen to optimize theeconomics of the process and will depend upon the specific dialkylcarbonate and diol being produced.

[0068] In another embodiment, at least a portion of the bottoms streamfrom diol product distillation column 22 can be fed to an evaporator(not shown) from which a cyclic carbonate-rich stream is recovered as avaporous effluent and recycled to transesterification reactor 11. In thecase of ethylene carbonate, the operating conditions of the evaporatortypically include temperatures in the range of about 120° C. to 180° C.,under a vacuum in the range of about 10 to 80 mm Hg. A liquid effluentstream from the evaporator, rich in catalyst, is also recovered andrecycled to carbonation reactor 1.

[0069] The second embodiment of the integrated process, which utilizes acirculating carbonation catalyst and a heterogeneous transesterificationcatalyst, is accomplished as described above with reference to FIG. 1,however the second aspect includes several process conditionmodifications. The foregoing description of FIG. 1 applies equally tothe description of FIG. 2 except that the following process conditionmodifications are specific to the carbonation catalyst—heterogeneoustransesterification catalyst process depicted in FIG. 2.

[0070] Referring now to FIG. 2, as in the first aspect, reactor 1 ischarged with alkylene oxide and catalyst via conduit 2 and with CO2 viaconduit 3. In the case of ethylene carbonate, the reaction of ethyleneoxide and CO2 is exothermic and the temperature of the reaction zone inthe second aspect of the invention is usually maintained below about220° C. and the pressure is maintained in the range from about 3448 kPa(500 psi) to about 6897 kPa (1000 psi) to enhance product quality.Preferably, the reaction temperature is between about 180° C. and 200°C.

[0071] In the second aspect, the liquid effluent from separator 7 ispassed via conduit 10 into transesterification reactor 11, which ispreferably a fixed-bed reactor. The WHSV in the fixed-bed reactor of thesecond aspect is generally about 0.3 to 3 hr−1.

[0072] As in the first aspect, a hydrolysis reactor can also beincorporated into the integrated process of the second aspect to providea highly purified diol, e.g., ethylene glycol. The feed to thishydrolysis reactor can include the diol product stream, which maycontain small amounts of cyclic carbonate, and some or all of theunreacted cyclic carbonate containing stream, that is otherwise recycledto the transesterification reactor. Water is also fed to the hydrolysisreactor to convert cyclic carbonate to diol with the formation of CO2by-product.

[0073] Use of the integrated process of all aspects of the presentinvention for the production of dimethyl carbonate and ethylene glycolis particularly well suited for incorporation into an ethylene glycolplant, which produces ethylene glycol from ethylene, oxygen and water bythe method described in the Encyclopedia of Chemical Processing andDesign, J. J. McKetta, Marcel Dekker, Inc., N.Y., pp. 237 to 243 (1984),which is incorporated herein by reference. Such a process is depicted inFIG. 3, wherein ethylene is first reacted with oxygen in a selectiveoxidation reactor 51 to produce ethylene oxide, CO2 and water (i.e., agas mixture) which are removed from reactor 51 via conduit 53. This gasmixture typically includes unconverted ethylene and one or morerelatively insert ‘ballast’ components, such as, methane. The gasmixture containing ethylene oxide is fed via conduit 53 to absorber unit55 where it is contacted with H2O fed via conduit 57, to absorb most ofthe ethylene oxide in the water. The ethylene oxide in water is taken asbottoms from absorber unit 55 via conduit 59 to distillation column 61wherein ethylene oxide is taken overhead from column 61 via conduit 2and fed to carbonation reactor 1 as discussed above in FIG. 1, and H2O,ethylene glycol, polyglycols and residual ethylene oxide are removed asbottoms via conduit 63 to join the feed to hydration reactor 73.Additional water 71 may be added to the feed of hydration reactor 73, toadjust the water to ethylene oxide ratio to a preferred range of betweenabout 10:1 to 30:1. Hydration reactor 73 preferably contains nocatalyst, and operates at 120° C. to 250° C. and pressure sufficient tokeep the components in the liquid phase. Products from hydration reactor73 are taken via conduit 78 to glycol separations unit 79, which removeswater and produces an ethylene glycol product stream 74, and residualpolyglycol stream 80. Optionally, the various polyglycols, such as,ethylene glycol and triethylene glycol, may be produced as separateproduct streams.

[0074] The diol product stream from column 22, which may contain up toabout 20% by weight ethylene carbonate, is taken via conduit 23 tohydrolysis unit 72. H2O is added to this unit, and most of the ethylenecarbonate entering the unit is hydrolyzed to ethylene glycol, which isremoved via conduit 77, and to CO2, which is removed via conduit 76.This CO2 may, optionally, be recycled to carbonation reactor 1.

[0075] The overhead from absorber unit 55 which includes ethylene, CO2and ballast gas is fed to CO2 separation unit 65 via conduit 67 whereinCO2 is taken overhead from CO2 separation unit 65 via conduit 71 whereinit is either released to the atmosphere or fed to carbonation reactor 1via conduit 3, and residual gas taken via conduit 69 is recycled toselective oxidation reactor 51.

[0076] The removed ethylene oxide can be returned as ethylene glycol,essentially on a 1:1 mole basis. Moreover, since the ethylene glycolproduced in accordance with the present invention is primarilymono-ethylene glycol, the amount of ethylene oxide that becomesmono-ethylene glycol as opposed to polyethylene glycols can be actuallygreater via the integrated process than by the ethylene glycol plant. Assuch, the ethylene glycol, having a higher percentage of mono-ethyleneglycol, can be returned to the ethylene glycol plant just prior to thepurification equipment, reducing the amount of glycol purificationrequired for the volume returned. This is especially beneficial for theproduction of fibergrade mono-ethylene glycol, which has a minimumpurity specification of 99.9 wt % mono-ethylene glycol.

[0077] Substantially pure dialkyl carbonate (e.g., dimethyl carbonate)is taken as bottoms via conduit 20 from distillation column 18 and fed,for example, into diphenyl carbonate production unit 81 having at leastone reactor with metal-containing catalyst operating at 80° C.-300° C.and 2-4,000 kPa, and having associated separation devices. This diphenylcarbonate production unit 81 produces diphenyl carbonate and methanol.The methanol is taken overhead via conduit 85 and recycled via conduit12 to transesterification reactor 11. The diphenyl carbonate is removedfrom diphenyl carbonate production unit 81 as bottoms stream via conduit83. The diphenyl carbonate is thereafter reacted within polycarbonatereactor 85 with bisphenol-A, thereby producing polycarbonate and phenol.The polycarbonate is removed as a sidestream via conduit 89 and sent tostorage, and the phenol is taken overhead via conduit 91 and recycled todiphenyl carbonate production unit 81.

[0078] It is contemplated that the unique integrated process forproducing dialkyl carbonate from alkylene and an oxygen-containing gascan be used cost effectively to produce commercial quantities of otherproducts, such as, furazolidone, agrochemical/pharmaceuticalintermediates, allyl diglycol carbonate (e.g., eye glass lenses), linearalkylcarbonate lubricants, oligocarbonate, light emitting diodes foroutdoor display panels and gasoline octane improver.

EXAMPLE 1

[0079] Set forth below are data from two commercial processes for themanufacture of dimethyl carbonate, as well as a sample of the crudedimethyl carbonate product produced by the unique low corrosiveintegrated process according to the present invention. Sample 1 belowwas generated from a standard process which employs a chloride coppercatalyst, as well as a continuous HCl injection or mitigation step tomaintain catalyst activity. Sample 2 was also generated from a standardprocess which employs a chloride copper catalyst, as well as acontinuous HCl injection or mitigation step to maintain catalystactivity.

[0080] The analytical results are set forth below in Table 1: TABLE 1SAMPLE 1 SAMPLE 2 GC Purity 99.83 100 TAN 0.0427 mg/g 0.0996 mg/g KOHKOH KF Water 113 ppm 79.4 ppm % Cl (XRF) 12 ppm 7 ppm Metals (ICP) Cd =0.0068 ppm Cr = 0.0088 ppm Co = 0.0060 ppm Fe = 0.0481 ppm Cu = 0.0091ppm K = 0.35 ppm Fe = 0.0767 ppm Mo = 0.020 ppm K = 0.98 ppm Ni = 0.018ppm Mg = 0.040 ppm Mn = 0.0046 ppm Na = 0.378 Zn = 0.0084 ppm AppearanceWater-white Water-white

[0081] As set forth above in Table 1, Samples 1 and 2 exhibit greaterthan 5 ppm of Cl which is extremely undesirable and corrosive to thedimethyl carbonate. The higher the level of chlorine, the greater riskof corrosion, i.e., dimethyl carbonate reacting on the reaction vesselsand conduits such that there is a substantial increase of metal contentin the crude dimethyl carbonate product.

What is claimed is:
 1. An integrated process for the production of a dialkyl carbonate and a diol from an alkylene oxide, carbon dioxide and an aliphatic monohydric alcohol comprising: (a) reacting an alkylene oxide with carbon dioxide in the presence of a halogen-free carbonation catalyst at a temperature in the range of about 50° C. to 250° C. and at a pressure of at least about 1379 kPa (200 psi) to provide a crude cyclic carbonate stream comprising a cyclic carbonate and said catalyst; and (b) reacting said cyclic carbonate with said aliphatic monohydric alcohol in the presence of said catalyst to provide a crude product stream comprising said dialkyl carbonate and said diol, wherein said crude product stream exhibits a halogen concentration of about 5 ppm or less.
 2. The process of claim 1, wherein said alkylene oxide is of the formula:

wherein R₁ and R₂ independently of one another denote a divalent group represented by the formula —(CH₂)_(m)—, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group, wherein R₁ and R₂ can share the same substituent; and said aliphatic monohydric alcohol is of the formula: R₃—OH wherein R₃ is an aliphatic C₁-C₁₂ hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group.
 3. The process of claim 1, wherein said halogen-free carbonation catalyst is selected from the group consisting of [1,1′(1-butylbenzimidazol-2yl)pentane]copper(II) di(trifluoromethanesulfonate), and hydroxides, carbonates or bicarbonates of quaternary ammonium bases.
 4. The process of claim 1, wherein said halogen concentration is about 2 ppm or less.
 5. The process of claim 1, wherein said pressure is in the range of about 3448 kPa to 6897 kPa (500 to 1000 psig) and the temperature is in the range of about 150° C. to 200° C.
 6. The process of claim 1, wherein the molar ratio of CO2 to alkylene oxide is in the range from about 1.05 to 1.15 and the molar ratio of aliphatic monohydric alcohol to cyclic carbonate is in the range from about 2:1 to 6:1.
 7. The process of claim 1, wherein said crude cyclic carbonate stream further comprises glycol impurities in an amount of up to 40% by weight, based upon total weight of said crude cyclic carbonate stream.
 8. The process of claim 7, wherein said cyclic carbonate is ethylene carbonate, said aliphatic monohydric alcohol is methanol, and said glycol impurities comprise ethylene glycol and higher molecular weight glycols.
 9. The process of claim 1, wherein said aliphatic monohydric alcohol contains dialkyl carbonate in an amount of up to 40% by weight, based upon the total weight of said aliphatic monohydric alcohol and said dialkyl carbonate.
 10. The process of claim 1, further comprising a step of recovering said dialkyl carbonate and said diol from said crude product stream.
 11. The process of claim 1, further comprising: (i) separating a first recycle stream comprising unreacted aliphatic monohydric alcohol from said crude product stream; (ii) recycling said first recycle stream to transesterification step (b); (iii) separating a second recycle stream comprising unreacted cyclic carbonate and said catalyst from said crude product stream; and (iv) recycling at least a portion of said second recycle stream to said carbonation step (a) and/or at least a portion of said second recycle stream to said transesterification step (b).
 12. The process of claim 2, wherein said cyclic carbonate is ethylene carbonate and said aliphatic monohydric alcohol is methanol.
 13. The process of claim 1, wherein said transesterification step (b) occurs in a reaction vessel selected from the group consisting of: a reactive distillation column, a distillation column with at least a plurality of reaction zones, a distillation column with a plurality of reaction zones having heat exchangers disposed between the distillation column and each reaction zone, and a distillation column with a plurality of reaction zones wherein bottoms thereof are optionally recycled to the distillation column.
 14. An integrated process for the production of a dialkyl carbonate and a diol from an alkylene oxide, carbon dioxide and an aliphatic monohydric alcohol comprising: (a) reacting an alkylene oxide with carbon dioxide in the presence of a halogen-free carbonation catalyst to provide a crude cyclic carbonate stream comprising a cyclic carbonate and said catalyst; and (b) reacting said cyclic carbonate and said halogen-free carbonation catalyst with said aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a crude product stream comprising said dialkyl carbonate and said diol, wherein said crude product exhibits a halogen concentration of about 5 ppm or less.
 15. The process of claim 14, wherein said alkylene oxide is of the formula:

wherein R₁ and R₂ independently of one another denote a divalent group represented by the formula —(CH₂)_(m)—, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group, wherein R₁ and R₂ can share the same substituent; and said aliphatic monohydric alcohol is of the formula: R₃—OH wherein R₃ is an aliphatic C₁-C₁₂ hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group.
 16. The process of claim 14, wherein said halogen-free, carbonation catalyst is selected from the group consisting of [1,1′(1−butylbenzimidazol-2yl)pentane]copper(II) di(trifluoromethanesulfonate), and hydroxides, carbonates or bicarbonates of quaternary ammonium bases.
 17. The process of claim 14, wherein said halogen concentration is about 2 ppm or less.
 18. The process of claim 14, wherein said transesterification catalyst is at least one catalyst selected from the group consisting of: anion exchange resins, inorganic metal oxides and inorganic solid support catalysts containing metals, and compounds or complexes of at least one element of groups 1, 2, 4-10, or 12-17 of the periodic table.
 19. The process of claim 14, wherein said crude cyclic carbonate stream further comprises glycol impurities in an amount of from about 0.5 to 40% by weight, based upon the total weight of said crude cyclic carbonate stream.
 20. The process of claim 19, wherein said cyclic carbonate is ethylene carbonate, said aliphatic monohydric alcohol is methanol, and said glycol impurities comprise ethylene glycol and higher molecular weight glycols.
 21. The process of claim 14, wherein said aliphatic monohydric alcohol further comprises dialkyl carbonate in an amount of up to 15% by weight, based upon the total weight of said aliphatic monohydric alcohol and said dialkyl carbonate.
 22. The process of claim 14, further comprising: (i) separating a first recycle stream comprising unreacted aliphatic monohydric alcohol from said crude product stream; (ii) recycling said first recycle stream to transesterification step (b); (iii) separating a second recycle stream comprising unreacted cyclic carbonate and said homogeneous carbonation catalyst from said crude product stream; and (iv) recycling at least a portion of said second recycle stream to carbonation step (a) and at least a portion of said second recycle stream to said transesterification step (b).
 23. The process of claim 15, wherein said cyclic carbonate is ethylene carbonate and said aliphatic monohydric alcohol is methanol.
 24. The process of claim 14, wherein said transesterification step (b) occurs in a reaction vessel selected from the group consisting of: a reactive distillation column, a distillation column with at least a plurality of reaction zones, a distillation column with a plurality of reaction zones having heat exchangers disposed between the distillation column and each reaction zone, and a distillation column with a plurality of reaction zones wherein bottoms thereof are optionally recycled to the distillation column.
 25. An integrated process for the production of a dialkyl carbonate and a diol from an alkylene which comprises: (a) reacting said alkylene with an oxygen-containing gas, thereby producing an alkylene oxide, carbon dioxide, and water; (b) reacting at least a portion of said alkylene oxide with said carbon dioxide in the presence of a halogen-free carbonation catalyst at a temperature in the range of about 50° C. to 250° C. and at a pressure of at least about 1379 kPa (200 psig) to provide a crude cyclic carbonate stream comprising a cyclic carbonate and said catalyst; and (c) reacting said cyclic carbonate with an aliphatic monohydric alcohol in the presence of said catalyst to provide a crude product stream comprising said dialkyl carbonate and said diol, wherein said crude product stream exhibits a halogen concentration of about 5 ppm or less.
 26. The process of claim 25, wherein said alkylene oxide is of the formula:

wherein R₁ and R₂ independently of one another denote a divalent group represented by the formula —(CH₂)_(m)—, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group, wherein R₁ and R₂ can share the same substituent; and said aliphatic monohydric alcohol is of the formula: R₃—OH wherein R₃ is an aliphatic C₁-C₁₂ hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group.
 27. The process of claim 25, wherein said halogen-free carbonation catalyst is selected from the group consisting of [1,1′(1−butylbenzimidazol-2yl)pentane]copper(II) di(trifluoromethanesulfonate), and hydroxides, carbonates or bicarbonates of quaternary ammonium bases.
 28. The process of claim 25, wherein said halogen concentration is about 2 ppm or less.
 29. The process of claim 25, wherein said pressure is in the range of about 3448 kPa to 6897 kPa (500 to 1000 psig) and the temperature is in the range of about 150° C. to 200° C.
 30. The process of claim 25, wherein a molar ratio of said carbon dioxide to said alkylene oxide is in the range from about 1.05 to 1.10 and a molar ratio of said aliphatic monohydric alcohol to said cyclic carbonate is in the range from about 2:1 to 6:1.
 31. The process of claim 25, wherein said crude cyclic carbonate stream further comprises glycol impurities in an amount of up to 40% by weight, based upon total weight of said crude cyclic carbonate stream.
 32. The process of claim 31, wherein said cyclic carbonate is ethylene carbonate, said aliphatic monohydric alcohol is methanol, and said glycol impurities comprise ethylene glycol and higher molecular weight glycols.
 33. The process of claim 25, wherein said aliphatic monohydric alcohol contains said dialkyl carbonate in an amount of up to 40% by weight, based upon the total weight of said aliphatic monohydric alcohol and said dialkyl carbonate.
 34. The process of claim 25, further comprising the further step of recovering said dialkyl carbonate and said diol from said crude product stream.
 35. The process of claim 25, further comprising: (i) separating a first recycle stream comprising unreacted aliphatic monohydric alcohol from said crude product stream; (ii) recycling said first recycle stream to the transesterification step (c); (iii) separating a second recycle stream comprising unreacted cyclic carbonate and said catalyst from said crude product stream; and (iv) recycling at least a portion of said second recycle stream to carbonation step (b) and/or at least a portion of said second recycle stream to said transesterification step (c).
 36. The process of claim 26, wherein said alkylene is ethylene, said cyclic carbonate is ethylene carbonate and said aliphatic monohydric alcohol is methanol.
 37. The process of claim 25, wherein said transesterification step (c) occurs in a reaction vessel selected from the group consisting of: a reactive distillation column, a distillation column with at least a plurality of reactors, a distillation column with a plurality of reactors having heat exchangers disposed between the distillation column and each reactor, and a distillation column with a plurality of reactors wherein bottoms thereof are optionally recycled to the distillation column.
 38. An integrated process for the production of a dialkyl carbonate and a diol from an alkylene which comprises: (a) reacting at least a portion of said alkylene with an oxygen-containing gas, thereby producing an alkylene oxide, carbon dioxide, and water; (b) reacting at least a portion of said alkylene oxide with said carbon dioxide in the presence of a halogen-free carbonation catalyst at a temperature in the range of about 50° C. to 250° C. and at a pressure of at least about 1379 kPa (200 psig) to provide a crude cyclic carbonate stream comprising a cyclic carbonate and said carbonation catalyst; and (c) reacting said cyclic carbonate and said carbonation catalyst with an aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a crude product stream comprising a dialkyl carbonate and diol, wherein said crude product stream exhibits a halogen concentration of about 5 ppm or less.
 39. The process of claim 38, wherein said alkylene oxide is of the formula:

wherein R₁ and R₂ independently of one another denote a divalent group represented by the formula —(CH₂)_(m)—, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group, wherein R₁ and R₂ can share the same substituent; and said aliphatic monohydric alcohol is of the formula: R₃—OH wherein R₃ is an aliphatic C₁-C₁₂ hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C₁-C₁₀ alkyl group and a C₆-C₁₀ aryl group.
 40. The process of claim 39, wherein said carbonation catalyst is selected from the group consisting of [1,1′(1-butylbenzimidazol-2yl)pentane]copper(II) di(trifluoromethanesulfonate), and hydroxides, carbonates or bicarbonates of quaternary ammonium bases.
 41. The process of claim 39, wherein said halogen concentration is about 2 ppm or less.
 42. The process of claim 39, wherein said transesterification catalyst is at least one catalyst selected from the group consisting of: anion-exchange resins, inorganic metal oxides and inorganic solid support catalysts containing metals, and compounds or complexes of at least one element of groups 1, 2, 4-10, or 12-17 of the periodic table.
 43. The process of claim 39, wherein said transesterification catalyst comprises a transitional alumina.
 44. The process of claim 39, further comprising: (i) separating a first recycle stream comprising unreacted aliphatic monohydric alcohol from said crude product stream; (ii) recycling said first recycle stream to the transesterification step (c); (iii) separating a second recycle stream comprising unreacted cyclic carbonate and said carbonation catalyst from said crude product stream; and (iv) recycling at least a portion of said second recycle stream to the carbonation step (b) and at least a portion of said second recycle stream to said transesterification step (c).
 45. The process of claim 39, wherein said cyclic carbonate is ethylene carbonate and said aliphatic monohydric alcohol is methanol.
 46. The process of claim 39, wherein said transesterification step (c) occurs in a reaction vessel selected from the group consisting of: a reactive distillation column, a distillation column with at least a plurality of reactors, a distillation column with a plurality of reactors having heat exchangers disposed between the distillation column and each reactor, and a distillation column with a plurality of reactors wherein bottoms thereof are optionally recycled to the distillation column.
 47. A process for producing polycarbonate which comprises the following steps: (a) reacting an alkylene with an oxygen-containing gas, thereby producing an alkylene oxide, carbon dioxide, and water; (b) reacting at least a portion of said alkylene oxide with said carbon dioxide in the presence of a halogen-free carbonation catalyst at a temperature in the range of about 50° C. to 250° C. and at a pressure of at least about 1379 kPa (200 psig) to provide a crude cyclic carbonate stream comprising a cyclic carbonate and said carbonation catalyst; (c) reacting said cyclic carbonate with an aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a crude product stream comprising a dialkyl carbonate and diol, wherein said crude product stream has a halogen concentration of about 5 ppm or less; (d) separating said dialkyl carbonate from said diol, thereby producing a dialkyl carbonate-enriched stream; (e) reacting said dialkyl carbonate-enriched stream and phenol in the presence of a metal-containing catalyst at a temperature in the range between about 80° C. to 300° C. and at a pressure in the range between about 2 kPa to 4000 kPa (absolute pressure), thereby producing diphenyl carbonate and alkanol; (f) separating said diphenyl carbonate from said alkanol, thereby producing a diphenyl carbonate-enriched stream; (g) reacting said diphenyl carbonate-enriched stream with bisphenol-A, thereby producing polycarbonate and phenol; and (h) separating said polycarbonate from said phenol, thereby producing a polycarbonate-enriched stream.
 48. A process for producing polycarbonate which comprises the following steps: (a) reacting an alkylene with an oxygen-containing gas, thereby producing an alkylene oxide, carbon dioxide, and water; (b) reacting at least a portion of said alkylene oxide with said carbon dioxide in the presence of a halogen-free carbonation catalyst at a temperature in the range of about 50° C. to 250° C. and at a pressure of at least about 1379 kPa (200 psig) to provide a crude cyclic carbonate stream comprising a cyclic carbonate and said carbonation catalyst; (c) reacting said cyclic carbonate and said carbonation catalyst with an aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a crude product stream comprising a dialkyl carbonate and diol, wherein said crude product stream has a halogen concentration of about 5 ppm or less; (d) separating said dialkyl carbonate from said diol, thereby producing a dialkyl carbonate-enriched stream; (e) reacting said dialkyl carbonate-enriched stream and phenol in the presence of a metal-containing catalyst at a temperature in the range between about 80° C. to 300° C. and at a pressure in the range between about 2 kPa to 4000 kPa (absolute pressure), thereby producing diphenyl carbonate and alkanol; (f) separating said diphenyl carbonate from said alkanol, thereby producing a diphenyl carbonate-enriched stream; (g) reacting said diphenyl carbonate-enriched stream with bisphenol-A, thereby producing polycarbonate and phenol; and (h) separating said polycarbonate from said phenol, thereby producing a polycarbonate-enriched stream. 